This invention relates to synthetic iron oxides. More particularly, the invention relates to synthetic red iron oxides prepared from soluble iron salts. More particularly, the invention relates to a method of improving the calcining effect when synthetic red iron oxide is prepared from soluble iron salts.
Iron oxide, either natural or synthetic, is a well-known pigment. Among the synthetic oxides are black, yellows, browns, tans, and reds. The black and brown oxides contain varying ratios of ferrous and ferric oxides. Yellow iron oxides are the monohydroate of ferric oxide. Tan oxides are a new class of iron oxide pigments, in which iron oxide is reacted with other metallic oxides to form a series of chemical compounds known as ferrites. For example, zinc ferrite theoretically contains 33.7% zinc oxide, but in practice from about 32% to about 34%, with the remainder being ferric oxide. Magnesium ferrite contains about 20% magnesium oxide. Synthetic red oxides can be made by 4 different methods. "Ferrite reds" are made by dehydrating the yellow oxide, thus converting the ferric oxide monohydrate to Fe.sub.2 O.sub.3. Another method is to precipitate black or brown oxide and then calcine at red heat to the ferric oxide state. The third method gives what is often referred to as "precipitated reds", made by direct precipitation from a solution of ferrous salts. The fourth method makes roasted "copperas reds" . Copperas (FeSO.sub.4.7H.sub.2 O) is purified to remove other metallic salts and is then dehydrated to the monohydrate (FeSO.sub.4.H.sub.2 O). The monohydrate is calcined (roasted) to give red alpha Fe.sub.2 O.sub.3 (copperas red). The present invention is primarily concerned with improving the color characteristics of roasted copperas reds but is applicable to other synthetic iron oxides. "Copperas reds" are chosen as the preferred embodiment.
Prior art inventions in the field of iron oxide technology are exemplified by U.S. Pat. Nos. 2,184,738, 2,394,579, 2,416,138, 2,620,261 and 3,009,821.
As mentioned above, copperas reds are typically prepared by calcining ferrous sulfate monohydrate, obtained from copperas. The calcining conditions are important for the color of the finished synthetic red iron oxides, particularly in view of the fact that in a commercial operation, a series of different distinct reds ranging from a light red to a dark purple must be produced consistent with established color standards. The main parameter for controlling the desired shade of red to be produced is the calcining temperature; the lighter reds, which are desirable products, are made at lower temperatures. To produce the darker red product, the temperature is increased in distinct steps. It is noted, with existing commercial calcining equipment, that the production of the lighter shades of reds requires temperatures which are so low as to result in an incomplete reaction of the ferrous sulfate monohydrate. In other words, when producing light reds, the calcined product contains a high percentage of unreacted material, which, since it has to be removed in the washing step, represents a material loss. Moving to the darker shades of red by increasing the calcining temperature, these losses become subsequently less with each step towards the next darker shade of red.
Based on these observations, it becomes desirable to be able to produce the lighter shades of red at higher temperatures, thus reducing losses of raw materials and subsequent environmental problems.
The calcining effect involves two results: the shade of the red product produced and the amount of uncalcined material remaining in the product. A higher calcination temperature reduces the loss of raw material (uncalcined product), but it also forms a darker red product. If a lower loss of material is obtained for a given red color - or, conversely, if a lighter red color of product is obtained for the same material loss, then the calcining effect is improved.
The improvement of the calcining effect leads to these two desirable features. It gives, first, an iron oxide of a lighter shade than presently produced, with the material losses still being within reasonable limits. Second, the iron oxide pigments resulting from the improved calcining effect have a higher tinting strength, which is desirable for the end-user, inasmuch as the tinting strength of a pigment is a measure of how much red pigment is needed to be mixed with a white pigment, such as TiO.sub.2, to arrive at a desired shade of color. Thus, a higher tinting strength means the end-user can produce a desired color with less red oxide pigment.